title: Biogeochemical controls on hexavalent chromium formation in estuarine sediments.
abstract: Predicting the aquatic and human health impacts of chromium (Cr) necessitates one to determine its speciation as either relatively nontoxic Cr(III) or toxic Cr(VI) and elucidate the influence of biogeochemical changes on its behavior and fate. In the Baltimore Harbor, Cr predominantly exists as Cr(III) associated with sediments. While reduction of Cr(VI) to Cr(III) is dominant in these anoxic sediments, the potential of Cr(III) oxidation and Cr(VI) reoccurrence during sediment resuspension and oxygenation resulting from dredging, bioturbation, and flood events poses a serious concern. In batch experiments, aqueous Cr(VI) spiked into continuously mixed anoxic suspensions was reduced to product Cr(III) under anaerobic conditions. No Cr(VI) reoccurrence was observed when conditions remained anaerobic. Aeration caused Cr(VI) reoccurrence from the abiotic oxidation of product Cr(III). Rates of aeration-driven Cr(VI) reoccurrence increased with pH, and Cr(VI) reoccurrence positively correlated with dissolved manganese (Mn) decline at pH ≥ 7. Aeration-driven oxidation of Mn(II) to Mn(III,IV)(hydr)oxides was the underlying mechanism causing product Cr(III) oxidation. Cr(VI) reoccurrence decreased with sediment loading and negatively correlated with the acid volatile sulfide (AVS) concentration. Although sediment resuspension and oxygenation may create temporary conditions conducive to Cr(VI) formation, long-term Cr(VI) persistence is unlikely in the presence of sediment reductants. While such natural attenuation in reducing environments mitigates the risk associated with Cr toxicity, this risk may still persist in Mn-rich and reductant-deficient environments.
authors.affiliation: Johns Hopkins University , Department of Geography and Environmental Engineering, 3400 N. Charles Street, 313 Ames Hall, Baltimore, Maryland 21218, United States.
authors.forename: Amar R
authors.initials: AR
authors.lastname: Wadhawan
authors.forename: Alan T
authors.initials: AT
authors.lastname: Stone
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Bouwer
mesh_terms: D015142:Baltimore; D002851:Chromatography, High Pressure Liquid; D002857:Chromium / Q000737:chemistry*; D063366:Estuaries; D019015:Geologic Sediments / Q000737:chemistry*; D013058:Mass Spectrometry; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D014874:Water Pollutants, Chemical; D002857:Chromium; C074702:chromium hexavalent ion